RSpec 3

Found that Travis was failing tests due to its default of using RSpec 3 (not sure when that behavior started, tho).  This PR:
- makes RSpec 3 explicit
- adds support for `its` and collection matchers (via `rspec-its` and `rspec-collection_matchers`)
- changes the `be_true` and `be_false` to `be_truthy` and `be_falsey`, and
- makes the `let(:klass)` in /spec/lib/damage/client_spec.rb to explicitly be `Damage::Client`

This last was weird... Not sure why it was failing.  Meh.